Density-dependent effects on nesting success of temperate-breeding Canada geese

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Density-dependent effects on reproduction can arise through variation in habitat quality or increased competition and interference among neighbours. Negative effects have been found in avian populations and these have been mainly attributed to food limitations. In this study, we investigated whether density-dependent effects could result from either heterogeneity in habitat suitability, interference among neighbours, or predation. To test these hypotheses, we collected data over eight years in a growing population of temperate-nesting Canada geese Branta canadensis maxima. We compared different parameters of nesting success of geese between two sites characterized by different nest densities and looked at the effects of nest proximity on these parameters within each site. At the landscape level, we found density-dependent effects due to variation in habitat quality associated with predation probabilities and flooding events. At a finer scale, nesting success declined with proximity to neighbours, probably due to increased aggressive interactions among pairs. However, complete clutch predation showed both positive and negative density-dependence, due to differences in predator community at each site. We concluded that density-dependent effects reduced nesting success of Canada geese through both heterogeneity in habitat safety and agonistic interference between neighbours but that density-dependent effects could also be positive in some instances.